1. Field of the Invention
This invention relates to an ultrasonic actuator using standing wave vibrations of longitudinal mode and bending mode, and more particularly to an ultrasonic actuator capable of efficiently driving a moving object without affecting the vibration characteristic of an elastic vibration member by extracting elliptic motions generated on one of opposite most slender planes of the vibration member shaped in a rectangular plate.
2. Description of the Prior Art
Standing wave vibrations or progressive wave vibrations of the natural mode are generated in an elastic vibration member being excited with an alternating current or vibrations having resonance frequency. Recently, these vibrations have been widely applied to mechanical-electric filters, various ultrasonic motors and so on.
The ultrasonic motors have commonly employed a single-mode or multi-mode of standing wave vibrations which are caused by expansion and contraction motions generated in the vibration member in the ratio unchanged from a local point of view.
As one example, there are bending mode, longitudinal mode and torsional mode in the standing wave vibrations. As illustrated in FIG. 1(A), the bending mode vibration is the periodic displacement motions which are generated (around the vibrational node N) in the opposite directions relative to the internal neutral plane Pn parallel to one excitation surface (broadest plane) of the vibration member 1. The longitudinal mode vibration is the expansion and contraction motions in the longitudinal (or width) direction of the vibration member 1 as shown in FIG. 1(B).
There has been proposed a "poking-type" ultrasonic motor using standing wave vibrations in Japanese Patent Publication No. SHO 58(1983)-32518(B2). This proposed ultrasonic motor is driven by the unidirectional expansion and contraction motions of the standing wave vibrations.
As a high-torque and high-efficient motor, ultrasonic motors utilizing torsional and longitudinal mode vibrations have been known. Also, in the commonly known actuator as shown in FIG. 2(A), the elastic vibration member 1 of a rectangular plate having opposite broadest surfaces 1a, 1b is provided on one of the broadest surfaces (excitation surface 1a) with the one vibrator 3 made of a piezoelectric material or the like to vibrate the vibration member. By pressing a plate-like moving object 5 against the other broadest surface 1b opposite to the excitation surface 1a of the vibration member 1 with considerably great force, the moving object is moved in one direction when the vibration member is excited to vibrate. In the drawings, reference numeral 9 denotes an exciting power source for applying to the vibrator 3 an alternating current having frequency resonant with the vibration member 1.
In a case of forcibly pressing a rotor 7 used as the moving object noted above against the vibration member 1 as shown in FIG. 2(B), the rotor 7 is rotated in one direction. In this ultrasonic motor, two vibrators 3 are attached one onto either side of the vibration member 1.
The standing wave motor utilizing the torsional and longitudinal vibrations has necessitated separate exciting power sources for inducing the respective modes of vibrations. In particular, the driving system (piezoelectric ceramic element polarized in the direction parallel to the electrodes) for inducing the torsional vibration different in structure from that of the longitudinal vibration has been required.
Although the conventional ultrasonic motors shown in FIGS. 2(A) and 2(B) are relatively simple in structure, these motors are disadvantageous in that both the moving objects 5 and 7 must be pressed against the vibration member 1 with considerably great force F. That is, the great pressing force imparted to the vibration member causes the resonant frequency characteristic inherent in the vibration member to be changed and/or undesirable bending vibrations to be induced. As a result, the conventional ultrasonic motors of this type have suffered a disadvantage such that large kinetic torque can in no way be brought about.